A metal material with a low melting point is cooled to a temperature at which a solid and liquid are both present, and in this state a carbon nanomaterial is kneaded with the low-melting metal material, to yield a composite material. A method for using a metal forming machine provided with heating means to inject and fill a die with the composite material and obtain a composite metal product is known, as disclosed in Japanese Patent Application Laid-Open Publication No. 2004-136363 (JP 2004-136363 A).
According to the composite forming method disclosed in JP 2004-136363 A, a molten low-melting metal material is cooled to a semi-molten state having a thixotropic property in which a liquid phase and a solid phase are both present. The low-melting metal material is kneaded in this state with a carbon nanomaterial, and a composite material is obtained. A metal forming machine provided with heating means is used to inject the composite material into a die while the thixotropic property is maintained; and a composite metal product is formed using the die.
Specifically, the method stated above is characterized in that the carbon nanomaterial is kneaded with the low-melting metal material in a semi molten state. Since the metal material is in a semi-molten state, the movement of the added carbon nanomaterial is limited, and the carbon nanomaterial can be prevented from rising or settling. As a result, a composite material of good quality can be obtained.
Of the liquid-phase portion and solid-phase portion that constitute the low-melting metal material in a semi-molten state, it is in the solid-phase portion that the carbon nanomaterial cannot be present. Therefore, the carbon nanomaterial composited by the method stated above is present in the liquid-phase portion.
If the added amount of the carbon nanomaterial is increased in order to improve functionality, the viscosity of the liquid-phase portion will increase, and the fluidity of the low-melting metal material in a semi-molten state will accordingly decline. A low-melting metal material in a semi-molten state of such description is harder to inject with a metal forming machine, and is not readily spread to all regions of the cavity of the die.
The technique mentioned above can be used when there is a small amount of carbon nanomaterial to be added, but cannot be used when the added amount is suitable.